Dislocation controlled wear in single crystal silicon carbide

For better design and durability of nanoscale devices, it is important to understand deformation in small volumes and in particular how deformation mechanisms can be related to frictional response of an interface in the regime where plasticity is fully developed. Here, we show that when the size of the cutting tool is decreased to the nanometer dimensions, silicon carbide wears in a ductile manner by means of dislocation plasticity. We present different categories of dislocation activity observed for single asperity sliding on SiC as a function of depth of cut and for different sliding directions. For low dislocation density, plastic contribution to frictional energy dissipation is shown to be due to glide of individual dislocations. For high dislocation densities, we present an analytical model to relate shear strength of the sliding interface to subsurface dislocation density. Furthermore, it is shown that a transition from plowing to cutting occurs as function of depth of cut and this transition can be well described by a macroscopic geometry-based model for wear transition.     

Modelling dislocation assisted tempering during rolling contact fatigue in bearing steels

Rolling contact fatigue in bearing steels is manifested by dark-etching regions, which are attributed to deformation induced tempering. In order to quantitatively explain this phenomenon, a model is suggested for martensite tempering assisted by dislocation glide during rolling contact fatigue. In the model, dislocations transport carbon from the matrix to carbide particles, provided that the carbon is located at a certain distance range from the dislocation contributing to the tempering process. By calculating the amount of carbon in the matrix, the kinetics of carbide thickening and hardness reduction are computed. It is found that the dark-etching region kinetics can be controlled by both bearing operation conditions (temperature and deformation rate) and microstructure (type, size, and volume fraction of carbides). The model is validated against tested bearings, and its limitations are discussed.     

A dislocation theory of isotropic polycrystalline plasticity

A theory of polycrystalline plasticity is developed in which the polycrystalline solid is modeled as an isotropic continuum. The rate of plastic deformation tensor is shown to be a function of the mobile dislocation density and the dislocation velocity vector summed over all active glide planes. The dislocation velocity vector is expressed in terms of the stress tensor and the normal vector to the dislocation glide plane. The condition of plastic incompressibility yields the fact that the dislocation glide planes are the octahedral shear planes of the stress tensor. As a special case the rate of plastic deformation tensor reduces to a relation analogous to the Prandtl-Reuss flow rule. The theory has been implemented in a 2-dimensional finite element code and two example problems are presented.     

Investigation of high-temperature plastic deformation using instrumented microindentation tests. Part IThe deformation of three aluminum alloys at 473 K to 833 K

Constant-load indentation tests were performed on wrought-2024, P/M-2024, and wrought-1100 aluminum alloys to assess the capability of the microindentation testing technique for measuring the high-temperature deformation rate controlling parameters of these alloys. The three alloys all display threshold indentation stress σ_th below which the indentation strain rate ε_ind approaches zero. The nominal inter-obstacle spacing, ℓ*, calculated from σ_th, increases with temperature in a way that is consistent with the known temperature dependence of the inter-particle spacing and dislocation cell size. The measured activation energy ΔG_o of ɛ_ind increases with temperature but remains within the range that is typical of deformation that occurs by dislocation glide limited by weak particles or dislocation/dislocation interactions. The three alloys tested show different trends of ΔG_o versus ℓ* and the trends are consistent with the known temperature dependence of the obstacles to dislocation glide. This study demonstrates that high-temperature indentation tests are sufficiently precise to detect changes in the operative deformation parameters between different alloys of the same general composition. This lays the groundwork for the use of this technique as a general tool for studying the local high-temperature deformation of a wide range of metal-based systems.     

Strain hardening in nanolayered thin films

Experimental results indicate that metal–ceramic multilayered thin films have unusual properties such as high strength, measurable plasticity and high strain hardening rate when both layers are nanoscale. Furthermore, the strength and strain hardening rate show a pronounced size effect, depending not only on the layer thickness but also on the layer thickness ratio. We analyze the strain hardening behavior of nanoscale multilayers using a three-dimensional crystal elastic–plastic model (3DCEPM) that describes plastic deformation based on the evolution of dislocation density in metal and ceramic layers according to confined layer slip mechanism. These glide dislocations nucleate at interfaces, glide inside layers and are deposited at interfaces that impede slip transmission. The high strain hardening rate is ascribed to the closely spaced dislocation arrays deposited at interfaces and the load transfer that is related to the layer thickness ratio of metal and ceramic layers. The measurable plasticity implies the plastically deformable ceramic layer in which the dislocation activity is facilitated by the interaction force among the deposited dislocations within interface and in turn is strongly related to the ceramic layer thickness.     

A comparative discrete-dislocation/crystal-plasticity analysis of bending of a single-crystalline microbeam

Bending of a micron-size single-crystalline beam is analyzed using both discrete-dislocation plasticity and crystal-plasticity formulations. Within the discrete-dislocation plasticity formulation, dislocations are treated as infinitely long straight-line defects residing within a linear elastic continuum. Evolution of the dislocation structure during bending is simulated by allowing the dislocations to glide in response to long-range interactions between different dislocations, and between dislocations and the applied stresses, and by incorporating various short-range reactions which can result in dislocation nucleation, annihilation or pinning. At each stage of bending, the stress and deformation fields are obtained by superposing the dislocation fields and the complementary fields obtained as a solution of the corresponding linear-elastic boundary value problem. The results obtained show that there is a continuing accumulation of geometrically necessary dislocations during bending which is expected due to the gradient in the strain throughout the beam height. In addition, it is found that localization of plastic flow into slip bands is a salient feature of materials deformation at the micron-length scale. Within the crystal-plasticity analysis, of beam bending, a small displacement gradient formulation is used and the material parameters selected in such a way that plastic flow localizes into deformation bands at low strains. It is found that, while the global response of the beam predicted by the two approaches can be quite comparable, fine details of the dislocation-based stress and deformation fields cannot be reproduced by the continuum crystal-plasticity model.     

On the rate sensitivity in discrete dislocation plasticity

Discrete dislocation dynamics simulations are carried out to systematically investigate the rate dependent deformation behaviour of polycrystalline bulk copper by varying the loading rate in the range of 100–25,000 s⁻¹ under tension. The underlying material model not only incorporates the realistic definition of nucleation time but also put emphasis on the role of obstacle density and their strength on dislocation motion. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. The numerical results show that the rate sensitivity of yield stress in bulk polycrystals is controlled by the density of Frank-Read sources, obstacles and their strength.     

A model on twinning-induced crack kinking out of a metal-ceramics interface

A continuum model is proposed to study the effects of deformation twinning on interface crack kinking in metal/ceramics layered materials. At the final stage of material failure, plastic work hardening exhausts and lattice rotation becomes main mechanism after competing with dislocation gliding. The crack-tip plasticity is established in terms of the second gradient of microrotation due to the coupling effect of the twins. The formed twinning structures not only shield the crack tip, but inhibit further dislocation emission by increasing the near-tip stress levels. A Dislocation-Free Zone (DFZ) can exist in the immediate vicinity of the tip. The model is based on the equivalence of the stresses derived from twin-based crack-tip plasticity, macroscopic plasticity and elasticity on the boundary. The two-parameter characterization of near-tip stress fields is used for the outer plastic zone to account for constraint effects. Crack kinking out of the interface follows the direction of the maximum flow stress from the crack-tip plasticity. The DFZ size and the crack-tip shielding ratio, as well as the kink angle, are obtained for various values of low hardening exponents and crack-tip constraints.     

先驱体转化法制备高性能碳化硼陶瓷材料研究进展

碳化硼(B₄C)是一种性能优良的特种陶瓷, 在军事、核工业、航空航天等领域有着广泛的应用。近年来,采用先驱体转化法制备碳化硼陶瓷得到了长足的发展。相比碳化硼材料的其它制备方法, 先驱体转化法具有元素组成简单、成型性好、陶瓷产率高、能耗低等优势, 在制备碳化硼粉体、纤维、介孔材料、微球等方面有着广泛的应用。本文综述了先驱体转化法制备碳化硼陶瓷的最新研究进展, 着重介绍了碳化硼先驱体的合成及应用, 并对先驱体转化法制备碳化硼陶瓷的发展方向和应用前景进行了展望。     

On the geometric origin of Orowan-type kinematic relations and the Schmid yield criterion

Summary It is shown that the Orowan-type kinematic relations as well as the Schmid yield criterion can be derived basing oneself on a formula defining the mean curvature of glide surfaces for a principal congruence of edge dislocation lines accompanied with a particular distribution of secondary point defects. Moreover, it appears that this mean curvature has the physical meaning of a mesoscopic material parameter defining a relation between the evolution of the dislocation state and plastic deformation. It is pointed out that the existence of Orowan-type relations puts kinematic constraints, dependent on the isometry group of glide surfaces, on the dislocation density tensor.     

Tensile elongation of lean-alloy austenitic stainless steels: transformation-induced plasticity versus planar glide

An Fe–13Cr–3.4Mn–0.47C lean-alloy stainless steel was made austenitic by solution annealing at 1250°C. Tensile tests between 20 and 200°C indicated enhancement of ductility at higher temperatures. At 200°C where planar glide, manifested as deformation twinning, was the dominant deformation mechanism, a uniform tensile elongation of 102% was achieved. At 20°C where deformation-induced α′-martensitic transformation replaced deformation twinning as the dominant deformation mechanism, tensile elongation was significantly impaired. The tensile elongation contribution by the planar glide was estimated to be at least four times that of the α′-TRIP (transformation-induced plasticity) mechanism. The results indicate that inexpensive lean-alloy austenitic stainless steels exhibiting pronounced α′-formation at room temperature could become highly formable at higher temperatures.     

Multiscale Modeling of Deformation in Polycrystalline Thin Metal Films on Substrates

The time‐dependent irreversible deformation of a thin metal film constrained by a substrate is investigated by a mesoscopic discrete dislocation simulation scheme incorporating information from atomistic studies of dislocation nucleation mechanisms. The simulations take into account dislocation climb along the grain boundaries in the film as well as dislocation glide along slip planes inclined and parallel to the film/substrate interface. The calculated flow stress and other features are compared with relevant experimental observations. The work is focused on deformation of a polycrystalline film without a cap layer, for which diffusive processes play an important role. The dislocation‐based simulations reveal information on the prevailing deformation mechanisms under different conditions and for different film thicknesses. Despite of the limitations of the two‐dimensional dislocation model, the simulations exhibit a film thickness dependent transition between creep dominated and dislocation glide dominated deformation, which is in good agreement with experimental observations.     

Gradient material mechanics: Perspectives and Prospects

This is a modest contribution dedicated to the work and virtue of George Weng, a prominent figure in material mechanics and a dear intellectual friend. The paper starts with the basics of gradient theory as applied to elasticity, plasticity and dislocation dynamics by introducing weak non-locality in the constitutive equations through Laplacian terms and corresponding deterministic internal lengths (ILs) characterizing the dominant deformation mechanisms. It then considers the interaction of such deterministic ILs with surface effects associated with internal or external surfaces, as well as stochastic effects associated with pre-existing or deformation-induced random microstructures. Experimentally observed stress drops and strain bursts are interpreted through combined gradient-stochastic models. Statistical features of corresponding deformation processes that cannot be fitted with Boltzmann–Gibbs–Shannon entropy statistics are interpreted by Tsallis q-entropy statistics. Some benchmark novel experiments for the direct determination of ILs for plasticity (by testing bulk specimens with gradient grain size) and dislocation dynamics (by testing thin films in TEM with gradient dislocation density) are proposed.     

Low-temperature in situ large strain plasticity of ceramic SiC nanowires and its atomic-scale mechanism

Large strain plasticity is phenomenologically defined as the ability of a material to exhibit an exceptionally large deformation rate during mechanical deformation. It is a property that is well established for metals and alloys but is rarely observed for ceramic materials especially at low temperature ( approximately 300 K). With the reduction in dimensionality, however, unusual mechanical properties are shown by ceramic nanomaterials. In this Letter, we demonstrated unusually large strain plasticity of ceramic SiC nanowires (NWs) at temperatures close to room temperature that was directly observed in situ by a novel high-resolution transmission electron microscopy technique. The continuous plasticity of the SiC NWs is accompanied by a process of increased dislocation density at an early stage, followed by an obvious lattice distortion, and finally reaches an entire structure amorphization at the most strained region of the NW. These unusual phenomena for the SiC NWs are fundamentally important for understanding the nanoscale fracture and strain-induced band structure variation for high-temperature semiconductors. Our result may also provide useful information for further studying of nanoscale elastic-plastic and brittle-ductile transitions of ceramic materials with superplasticity.     

Light element ceramics

An overview of a few structurally important light element ceramics is presented. Included in the overview are silicon nitride, sialon, aluminium nitride, boron nitride, boron carbide and silicon carbide. Methods of preparation, characterization and industrial applications of these ceramics are summarized. Mechanical properties, industrial production techniques and principal uses of these ceramics are emphasized. Contribution No. 76 from the Materials Research Laboratory     

Ultralarge Bending Strain and Fracture‐Resistance Investigation of Tungsten Carbide Nanowires

Hard tungsten carbide (WC) with brittle behavior is frequently applied for mechanical purposes. Here, ultralarge elastic bending deformation is reported in defect‐rare WC [0001] nanowires; the tested bending strain reaches a maximum of 20% ± 3.33%, which challenges the traditional understanding of this material. The lattice analysis indicates that the dislocations are confined to the inner part of the WC nanowires. First, the high Peierls–Nabarro barrier hinders the movement of the locally formed dislocations, which causes rapid dislocation aggregation and hinders long‐range glide, resulting in a dense distribution of the dislocation network. In this case, the loading is dispersed along multiple points, which is then balanced by the complex internal mechanical field. In the compressive part, the possible dislocations predominantly emerge in the (0001) plane and mainly slip along the axial direction. The disordered shell first forms at the tensile side and prevents the generation of nanocracks at the surface. The novel lattice kinetics make WC nanowires capable of substantial bending strain resistance. Analytical results of the force–displacement (F–d) curves based on the double‐clamped beam model exhibit an obvious nonlinear elastic characteristic, which originates fundamentally from the lattice anharmonicity under moderate stress.     

碳化硼抗弹陶瓷的制备方法及应用

碳化硼陶瓷具有高硬度、高熔点和低密度的特点,是优异的结构陶瓷,在民用、宇航和军事等领域都得到了重要应用。本文中综述了碳化硼结构陶瓷的优异性能和制备新方法,重点介绍了自蔓延高温合成法(SHS),碳管炉、电弧炉碳热还原法,激光化学气相反应法,溶胶-凝胶碳热还原法等合成碳化硼粉末的主要方法以及碳化硼成型和烧结的常用方法,简述了碳化硼抗弹陶瓷材料的发展应用和研究现状。     

Quasistatisches Verformungsverhalten reiner Sintereisen im Temperaturbereich von –184 bis 600°C

Quasistatic deformation behaviour of pure sintered iron in the temperature range between –184 and 600°C The deformation behaviour of pure sintered iron materials with densities between 6,88 und 7,57 g/cm³ was investigated in tension tests in the temperature range of –184 and 600°C. Supplementary compression tests were carried out at 20°C. Increasing density leads to increasing material resistances and ductility properties due to the increase of the bearing specimen cross sections as well as due to smaller numbers of pores, more spherical pores with smaller notch effects and smaller numbers of mircocracks, which are initiated at pores. After equal deformations, due to pore closing effects and the impediment of crack initation, the flow stresses of compressively deformed specimens are larger than those of tensily deformed. The deformation behaviour is dominated at low temperatures by thermal activated glide processes of dislocations and their interactions with short range obstacles, at middle temperatures by dynamic strain ageing due to elastic interactions of glide dislocations and diffusing carbon atoms and at high temperatures by recovery controlled dislocation creep processes.     

Structural Effects on the Strength of Ceramics

The key features of the processes underlying the failure of ceramics are considered for a wide temperature range. The brittleness and high-temperature plasticity of ceramics are correlated with their crystal chemistry. The general issues related to the strength of ceramics are treated in terms of synergetics, which deals with the spatiotemporal ordering and self-organization in nonequilibrium systems. The strength of ceramics is shown to be governed by its structure on different scales—from atomic to macroscopic. The conclusion is drawn that, in the strict sense, strength is not a property of the material; rather it characterizes its quality.     

Manipulation of matter by electric and magnetic fields: Toward novel synthesis and processing routes of inorganic materials

The use of external electric and magnetic fields for the synthesis and processing of inorganic materials such as metals and ceramics has seen renewed interest in recent years. Electromagnetic energy can be utilized in different ways to improve or accelerate phase formation and stabilization, chemical ordering, densification and coarsening of particle-based materials (pore elimination and grain growth), and mechanical deformation (plasticity and creep). In these new synthesis and processing routes, the resulting microstructures and macroscopic material behavior are determined by the interaction of the applied fields with defects such as single or clustered point defects, dislocation networks, and interfaces. Multiscale experimental investigations and modeling are necessary to unveil the mechanisms underlying this field-assisted manipulation of matter.